Polypropylene compositions have been known to be difficult to print, bind and paint due to their nonpolarity. Therefore, there have been proposed improved methods for providing stable paint-coatings on the molded articles of polypropylene compositions. For example, so-called primers or under-coatings are usually applied as a medium layer between the polypropylene compositions and the paints to improve their compatibility. This method is, however, still unsatisfactory from the industrial point of view because a primer itself is expensive, an additional step required to apply a primer increases the costs of the final products, and a primer poses an environmental problem associated with its waste treatment. Furthermore, the application of a primer is usually preceded by cleaning with vapors of halogenated hydrocarbons such as 1,1,1-trichloroethane. But, the use of 1,1,1-trichloroethane is now being banned in consideration of the ozone layer depletion and the protection of the global environment.
The surface treatments of molded articles of polypropylene compositions are another means to increase the binding strength of paints, prints or adhesives to them. For example, sandblasting, chromic acid treatment, flame treatment, corona discharge treatment, plasma treatment, surface activation with functional groups, and photo-induced surface grafting are applied before applying paints to the articles. However, none of them is satisfactory from the standpoint of performance and environmental problems.
Under the circumstances, the present inventors conducted research, which led to the finding that the compatibility and binding property of polypropylene compositions are greatly improved when crystalline polypropylene and an ethylene-propylene copolymer rubber are blended with a hydroxyl group-containing propylene oligomer. More specifically, it has been found that a polypropylene resin composition which comprises crystalline polypropylene, an ethylene-propylene copolymer rubber, a hydroxyl group-containing propylene oligomer, and optionally an inorganic filler has an advantageous and unique property in that it can easily and stably be paint-coated without pre-cleaning with vapors of 1,1,1-trichloroethane and a primer, but simply treating it with water or non-hologenated solvents such as alcohols, aromatic hydrocarbons or ketone-solvents before applying paints.